Part 1 - Basic of Material Selection Guide

As new engineers in the field of mechanical and piping engineering, it's important to understand why a certain material is selected for refinery projects [This is from the Material Department].

The key factor here is durability against corrosion over the project's lifespan.

Another aspect considered is the corrosion inhibition factor, which evaluates how certain materials can resist corrosion. With this data, material engineers make an informed decision between using carbon steel or a corrosion-resistant alloy (CRA).

Generally, carbon steel is a practical choice for many parts of a refinery project. However, in environments with high corrosion, where pipe cleanliness is crucial, or for smaller pipes, CRA becomes a preferable choice. It's also essential to select materials that are resistant to age-related issues like cracking.

This guide will introduce you to various materials that are commonly used in refinery facility projects, helping you to understand why certain material is selected instead of others.

Carbon Steel

Carbon steel is a common material in the oil and gas industry, but it's important to know that it's prone to various types of corrosion. While it offers moderate resistance to corrosion, it's often acceptable to use if the corrosion rate is controllable and can be regularly monitored for damage and extent.

However, in high-velocity conditions, carbon steel can corrode quickly. So, if used without any protective measures, it needs a larger corrosion allowance and extra consideration in design stresses to ensure safety and longevity. In the industry, there are many ways to maintain and mitigate these risks.

Despite its susceptibility to corrosion, carbon steel remains a popular choice due to its cost-effectiveness and wide availability, suitable for many applications in oil and gas.

When selecting materials, a primary consideration is how carbon steel will hold up against internal corrosion, like CO2 corrosion, in the operational environment. In this context, specialized corrosion software is used to estimate the corrosion rate and to calculate the necessary corrosion allowance for the expected lifespan of the project.

This corrosion allowance means adding extra thickness to the carbon steel to compensate for anticipated metal loss.

This Chapter touch a bit on the Corrossion Allowance considered in a Shell and Tube Heat Exchanger.

Unprotected carbon steel is also vulnerable to atmospheric and marine environments. When exposed to air and water, oxidation and further corrosion can occur. Other concerns include pitting and crevice corrosion.

To combat this, external protective coatings are widely used. The choice of coating should align with the latest Codes and Standards and requires systematic inspection and maintenance.

If carbon steel, even with additional corrosion allowance and inhibitors, can't withstand the corrosive environment over the design life, then it's time to consider a higher-grade, alloyed material.

Next we will look at other Steel including Low Alloy Steel and Austenitic Stainless Steel.